"Aw man, if I'd have known that Valve was going to release Left4Dead 2 this Fall, I never would have bought Left4Dead and had a load of fun playing a co-op zombie shooter with my friends this past year. I guess all that fun I had was a total waste of my time and money!"
How does Left4Dead 2 make Left4Dead obsolete? You can still play it and it still looks great. It's still fun. Just because the sequel comes out, nobody's going to play L4D anymore? People still play TFC. People still play Quake III. People will still play L4D. In droves.
I'd also like to point out that while people keep citing TF2 as the free support model Valve should be following, TF2 is probably the ONLY game that Valve gives free updates for. Where is the official free Portal content update? What about Half-Life 2? Episode 1 wasn't much more than extra areas and some new enemies. Same with Episode 2. Why didn't they give that to us for free as a Half-Life 2 expansion pack?
Is it just the time? Is it because it's coming out so soon? Aren't these the same people that complain about Valve taking forever to come out with new stuff? So now Valve gets a sequel to a very popular game out in about a year, and people are complaining? Honestly, there's no pleasing people.